The diamond particles obtained through the explosive-bombardment method (the impact method) wherein an impact pressure is caused using an oxygen-deficient explosive compound such as trinitrotoluene (TNT) or hexogen (RDX) have an extremely small primary particle size of 3 to 20 nm and accordingly, it has been referred to as nanodiamond (ND). However, a non-graphitic or graphitic film is welded onto the surface of such ND fine particles and the ND particles presently prepared are those in the form of secondary and/or tertiary agglomerates whose particle size falls within the range of from 50 to 7500 nm. Accordingly, they are also referred to as cluster diamond (CD) particles (see, for instance, Non-patent Documents 1 and 2 specified later). It has been expected that the ND particles may be used in wide variety of fields such as abrasives, lubricants, heat-exchanging fluid mediums, composite materials with resins and/or metals, electronic materials such as low dielectric films and emitter materials, medical fields such as DNA-carriers, virus-capturing carriers or the like, in addition to the usual applied fields of the diamond particles, because of their nano-order particle size.
Thus, when using, on an industrial scale, the ND particles in the form of composite materials, the ND particles must be provided in the form of a dispersion in which fine ND particles having a particle size ranging from a few to several hundreds of nanometer are dispersed in a liquid. However, when handling the ND particles in the form of a dispersion or a solution in which the particles having a particle size on the order of nanometer are dispersed, the particles are quite susceptible of undergoing agglomeration among them as the particle size thereof is reduced to a smaller level and the particles thus agglomerated are liable to cause settling. For this reason, it would be quite difficult to obtain a stable dispersion. Regarding the ND particles, there have thus variously been investigated a variety of methods for stably dispersing the primary ND particles per se in a liquid medium through the treatment of CD particles in an ultrasonic homogenizer or a beads-mill wet pulverizer (see, for instance, Patent Documents 1 and 2, specified below). However, the dispersions prepared according to these methods are insufficient in the long term stability and the particles present therein again undergo agglomeration after drying the same. Therefore, these conventional techniques have never met the commercial needs.
On the other hand, there has been reported a method for the reaction of CD particles with a fluorine gas for the purpose of the disintegration of any secondary and tertiary aggregates of CD particles. For instance, CD particles are brought into contact with fluorine gas at a reaction temperature ranging from 300 to 500° C. and a fluorine gas pressure of 0.1 MPa, for a reaction time ranging from 5 to 10 days, to thus give fluorinated CD particles having an F/C molar ratio of about 0.2 (as determined by the XPS technique and the elemental analysis), while maintaining the desired diamond structure thereof (see Non-Patent Document 3 specified below). The result of the TEM observation indicates that this fluorination treatment would permit the partial disintegration of the CD particles whose secondary particle size is about 40 μm and that the resulting disintegrated CD particles have a particle size on the order of about 200 μm. Moreover, it has been confirmed that the frictional coefficient of the CD particle is significantly reduced as is evident from the rotary type friction test using a mixed powder thereof with polytetrafluoroetylene (PTFE) (see Non-Patent Document 4 specified below). In this respect, it has been reported that the non-graphitic carbon present in the surface of the ND particles is removed due to the reaction at a high temperature and that the surface energy is reduced through the formation of groups such as CF groups, CF2 groups and/or CF3 groups on the ND particle surface, as is evident from the TEM observation of the ND particles, which indicates the presence of a clear lattice pattern thereof (see, Non-Patent Document 5 specified later). Moreover, it has likewise been reported that fluorinated ND particles whose fluorine content range from 5 to 8.6% by mass (as determined by the EDX analysis) could be synthesized by the fluorination reaction of ND particles carried out at a reaction temperature of 150, 310, 410 or 470° C., at a flow rate ratio: F2/H2 of 3:1 and a reaction time of 48 hours and there has also been reported such a result that the resulting fluorinated ND particles are improved in their solubility in a polar solvent such as ethanol as compared with the starting ND particles (see Non-Patent Document 6 and Patent Document 3, specified below).    Patent Document 1: JP-A-2005-1983;    Patent Document 2: JP-A-2005-97375;    Patent Document 3: U.S. Patent Application No. 2005/0158549 A1;    Non-patent Document 1: OSAWA Eiji, Bulletin of the Abrasive Grain-Procession Society of Japan, 2003, 47: 414;    Non-patent Document 2: HANADA Kotaro, Bulletin of the Abrasive Grain-Procession Society of Japan, 2003, 47: 422;    Non-patent Document 3: OI Tatsumi, YONEMOTO Akiko, KAWASAKI Shinji, OKINO Fujio, HIGASHIBARA Hidekazu, The Collected Main Purports of the 26th Meeting on Fluorine Chemistry, held on 2002, November;    Non-patent Document 4: YONEMOTO Akiko, OI Tatsumi, KAWASAKI Shinji, OKINO Fujio, KATAOKA Fumiaki, OSAWA Eiji, HIGASHIBARA Hidekazu, The Collected Resumes of the 83rd Annual Spring Meeting of the Chemical Society of Japan, held on 2003, March;    Non-patent Document 5: H. Touhara, K. Komatsu, T. Ohi, A. Yonemoto, S. Kawasaki, F. Okino and H. Kataura: Third French-Japanese Seminar on Fluorine in Inorganic Chemistry and Electrochemistry (April, 2003); and    Non-patent Document 6: Y. Liu, Z. Gu, J. L. Margrave, and V. Khabashesku; Chem. Mater. 16, 3924 (2004).